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“The Duke of Edinburgh is a funny man,” Brandreth wrote in The Radio Times. “He is the fellow who said, ‘If you ever see a man opening the car door for his wife, it’s either a new car or a new wife.’ He has been making the Queen laugh for more than 70 years.”

2. He is his own man

Since vowing at her 1953 coronation to be the Queen’s “liege man of life and limb,” Philip has dutifully fulfilled her role as the Queen’s consort – but that doesn’t mean he takes a back seat in their relationship.

“The Queen wears the crown, but her husband wears the trousers,” writes Brandreth. “He is the power behind the throne: steadfast, never-failingly supportive.”

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What’s more, “because she is the Queen, no one in the world treats Elizabeth II entirely normally – except for Prince Philip,” says the biographer. “Conversely, the Queen is the only person in the world who can say to the Duke of Edinburgh, ‘Oh Philip, do shut up.’ And she does.”

3. They accept each other’s differences

Philip, who “wants to do what he did when he was 55”, his grandson Peter Phillips recently said, “is dynamic, outgoing, adventurous, challenging,” observes Brandreth. The Queen, on the other hand, “is conservative, placid, content to do things as they have been done before. He reads a good deal, where she doesn’t. He has infinite patience. He has rather less . . . As a couple they are allies, but that doesn’t mean they don’t have their differences.”